Methods of making microelectronic assemblies including compliant interfaces

ABSTRACT

A method of making a microelectronic assembly including a compliant interface includes providing a first support structure, such as a flexible dielectric sheet, having a first surface and a porous resilient layer on the first surface of the first support structure, stretching the first support structure and bonding the stretched first support structure to a ring structure. The first surface of a second support structure, such as a semiconductor wafer, is then abutted against the porous layer and, desirably after the abutting step, a first curable liquid is disposed between the first and second support structures and within the porous layer. The first curable liquid may then be at least partially cured.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation of U.S. patent applicationSer. No. 09/517,852, filed Mar. 2, 2000, which is a divisional of U.S.patent application Ser. No. 08/842,313, filed Apr. 24, 1997, now U.S.Pat. No. 6,133,639, which in turn is a divisional of U.S. patentapplication Ser. No. 08/365,699, filed Dec. 29, 1994, now U.S. Pat. No.5,659,952, which in turn is a continuation-in-part of U.S. patentapplication Ser. No. 08/309,433, filed Sep. 20, 1994, the disclosures ofwhich are hereby incorporated by reference herein.

FIELD OF THE INVENTION

[0002] The present invention relates, generally, to providing acompliant interface for a semiconductor chip, and more particularlyrelates to a method and apparatus for providing a compliant interface toaccommodate for differences in the thermal coefficient of expansionmismatch between a semiconductor chip and a support structure, such as aprinted wiring board.

BACKGROUND OF THE INVENTION

[0003] In attempting to use the area on printed wiring boards moreefficiently, semiconductor chip manufacturers have recently beenswitching from larger, more cumbersome interconnection conventions, suchas pin grid arrays (“PGAs”) and the perimeter leaded quad flat packs(“QFPs”), to smaller conventions, such as ball grid arrays (“BGAs”).Using BGA technology, semiconductor chips are typically interconnectedto their supporting substrates using solder connections, such as with“flip-chip” technology. However, when solder alone is used tointerconnect the chip contacts to the substrate, the columns of solderare generally designed to be short to maintain the solder's structuralintegrity. This results in minimal elastic solder connection propertieswhich further results in increased susceptibility to solder cracking dueto the mechanical stress of the differential thermal coefficient ofexpansion (“TCE”) of the chip relative to the supporting substratethereby reducing the reliability of the solder connection. In otherwords, when the chip heats up during use, both the chip and thesubstrate expand; and when the heat is removed, both the chip and thesubstrate contract. The problem that arises is that the chip and thesubstrate expand and contract at different rates and at different times,thereby stressing the interconnections between them. As the features ofsemiconductor chips continue to be reduced in size, the number of chipspacked into a given area will be greater and the heat dissipated by theeach of these chips will have a greater effect on the thermal mismatchproblem. This further increases the need for a highly compliantinterconnection scheme for the chips.

[0004] The solder cracking problem is exacerbated when more than onesemiconductor chip is mounted in a package, such as in a multichipmodule. Multichip modules continue to grow in popularity; however, asmore chips are packaged together, more heat will be dissipated by eachpackage which, in turn, means the interconnections between a package andits supporting substrate will encounter greater mechanical stress due tothermal cycling. Further, as more chips are integrated into multichipmodules, each package requires additional interconnections therebyincreasing overall rigidity of the connection between the module and itssupporting substrate.

[0005] An interconnection solution put forth in U.S. Pat. No. 4,642,889,entitled “Compliant Interconnection and Method Therefor” issued toGrabbe seeks to alleviate the aforementioned solder cracking problem byembedding wires within each solder column to reinforce the solderthereby allowing higher solder pedestals and more elasticity. Anothersolution includes spirally wrapping wire around the outside of thesolder. A further solution put forth includes providing a combination ofsolder and high lead solder, as found in U.S. Pat. No. 5,316,788,entitled “Applying Solder to High Density Substrates” issued to Dibbleet al.

[0006] Still other prior art solutions make use of a underfill materialdisposed between the chip and the supporting substrate in an attempt toreduce the stress caused by TCE mismatch. Without the underfillmaterial, this stress is typically concentrated at the weakest part ofthe solder balls. The underfill material allows this stress to be moreuniformly spread out over the entire surface of the solder balls.Examples of the use of underfill materials may be found in U.S. Pat.Nos. 5,194,930, 5,203,076 and 5,249,101. All of these prior artsolutions are aimed at reducing the shear stress endured by theinterconnections caused by thermal cycling. However, each of thesesolutions also encounters significant problems such as insufficientcompliance and process cost.

[0007] Several inventions, commonly assigned to the assignee of thepresent invention, deal effectively, but specifically differently, withthe thermal cycling problem. For example, U.S. Pat. No. 5,148,266discloses improvements in semiconductor chip assemblies and methods ofmaking the same. As set forth in the '266 patent, a semiconductor chipcan be connected to a substrate using a sheet-like, and preferablyflexible, interposer. The interposer overlies the top, contact-bearingsurface of the chip. A first surface of the interposer faces towards thechip whereas a second surface faces away from the chip. Electricalterminals are provided on the second surface of the interposer, and theinterposer is provided with apertures extending through it. Flexibleleads extend through these apertures between contacts on the chip andthe terminals on the second surface of the interposer. The terminals canbe bonded to a substrate. Because the terminals are movable relative tothe contacts on the chip, the arrangements described in the '266 patentprovide excellent resistance to differential expansion of the chiprelative to the substrate caused by thermal cycling. The interposerdisclosed in the '266 patent may also include a compliant layer disposedbetween the terminals and the chip.

[0008] Copending, commonly assigned U.S. patent application No.08/123,882, filed Sep. 20, 1993, the disclosure of which is herebyincorporated herein by reference, discloses a method for creating aninterface between a chip and chip carrier including spacing the chip agiven distance above the chip carrier, and introducing a liquid in thegap between the chip and carrier. Preferably, the liquid is an elastomerwhich is cured into a resilient layer after its introduction into thegap. In another preferred embodiment, the terminals on a chip carrierare planarized or otherwise vertically positioned by deforming theterminals into set vertical locations with a plate, and a liquid is thencured between the chip carrier and chip.

[0009] Despite the positive results of the aforementioned commonly ownedinventions, still further improvements would be desirable.

SUMMARY OF THE INVENTION

[0010] The present invention provides a method and apparatus forproviding a compliant interface for semiconductor chips to accommodatefor the typically large thermal expansion mismatch between a chip andits support structure.

[0011] More specifically, one aspect of the present invention provides amethod of fabricating a compliant interface for a semiconductor chip,typically comprised of a compliant encapsulation layer having acontrolled thickness. A first support structure, such as a flexible,substantially inextensible dielectric film, having a surface isprovided. A porous resilient layer, such as a layer of a plurality ofcompliant pads, is attached to the first surface of the first supportstructure, any two adjacent compliant pads defining a channeltherebetween. Attaching the compliant pads to the first supportstructure may be accomplished a number of different ways. In oneembodiment, a stencil mask having a plurality of holes extendingtherethrough is placed on top of the first surface of the supportstructure. The holes in the mask are then filled with a curable liquidelastomer. Desirably, liquid elastomer has a thick enough consistency sothat the mask may then be removed before curing the elastomer. After themask has been removed, the elastomer is at least partially cured, suchas by heating or exposing to ultraviolet light. The filling step may beaccomplished by screening the liquid elastomer across an exposed surfaceof the mask such that the elastomer is deposited into the holes of themask. Other methods are also suitable, such as dispensing the elastomerinto each of the holes individually. Thus, there is provided an assemblywhich includes an array of compliant pads further having channelsbetween substantially all of the adjacent pads. At this stage of theprocess, the vertical height of the pads need not be extremely uniform.

[0012] In a further stage, the assembly including the array of pads isused with a second support structure. In one embodiment, the secondsupport structure is a semiconductor chip having a plurality of contactson a first surface. The first surface of the chip is abutted against thearray of compliant pads and the contacts are electrically connected to acorresponding plurality of terminals on a second side of the supportstructure. Typically, the first surface of the chip is pressed againstthe pads to compress them, thus ensuring the chip is uniformly supportedacross its first surface and further ensuring the planarity of the firstsupport structure, or flexible dielectric film, with respect to thefirst surface of the chip. Where the dielectric film has terminalsthereon, the terminals desirably are held coplanar with one anotherduring this step. For example, the dielectric film and the chip may beheld between a pair of opposed platens, so that the terminals bear onone platen and are brought into a substantially coplanar condition. Acompliant filler, such as a curable liquid elastomer, is then injectedinto the channels between the chip and the support structure and aroundthe compliant pads while the chip and support structure are held inplace. The elastomer may then be cured to form a substantially uniform,planar, compliant layer between the chip and the support structure.

[0013] In a further embodiment, the second support structure includes aplurality of chips. The array of compliant pads is large enough tooverly several chips simultaneously. This method may be used to form acompliant interface for multiple chips which then may either be cut intoindividual chips or may be used as a multi chip module. The above methodmay also be used to form a compliant interface for a semiconductor waferbefore the individual chips are separated. After the compliant interfaceis formed, the wafer may be cut into individual chips or into multi-chipmodules.

[0014] A further embodiment provides for an array of compliant pads heldin place using a holding element. The pads are then fixably placed toeither a surface of the chip or the supporting substrate. The holdingelement is then removed and the pads are compressed between the chip andthe supporting substrate. Liquid elastomer is injected into the channelsbetween the adjacent pads, as described in the above embodiments.

[0015] The pads or the pad/injected elastomer combination provide acompliant, planar interface which effectively accommodates for thethermal coefficient of expansion mismatch between the chip and asupporting substrate thereby alleviating much of the stress on theconnections therebetween. Further, the combination provides an effectiveencapsulation barrier against moisture and contaminants.

[0016] The foregoing and other objects and advantages of the presentinvention will be better understood from the following DetailedDescription of Preferred Embodiments, taken together with the attachedfigures.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIG. 1 shows a side view of one embodiment of a compliantsemiconductor chip interface having a plurality of compliant pads,according to the present invention.

[0018]FIG. 2 shows a side view of the embodiment shown in FIG. 1,wherein the channels between the compliant pads are filled with acurable liquid, according to the present invention.

[0019]FIGS. 3A and 3B show a side view of the mask and elastomer used toform the compliant pads, according to further embodiments of theinvention.

[0020] FIGS. 4A-C show a top plan view of several different pad shapesand registrations in relation to the terminal's positions, according tofurther embodiments of the invention.

[0021]FIGS. 5A and 5B show a perspective and a side view of the supportstructure and compliant pads, according to a further embodiment of thepresent invention.

[0022]FIG. 5C shows a side view of the embodiment shown in FIGS. 5A and5B, wherein multiple encapsulated chips have been abutted against thecompliant pads.

[0023]FIG. 6 shows a side view of a compliant interface being formedbetween a wafer and the support structure, according to a furtherembodiment of the present invention.

[0024]FIG. 7 shows a side view of a plurality of compliant pads disposedwithin holes in a holding element, according to a further embodiment ofthe present invention.

[0025]FIG. 8 shows a side view of a further embodiment of the inventionhaving a compliant interface between a heat spreader and a supportstructure.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0026]FIGS. 1 and 2 show a side view of a chip size package embodiment,according to the present invention. A support structure 100 is providedhaving a first and a second surface. The support structure 100 may besubstantially rigid, semi-rigid or flexible. However typically, thesupport structures 100 is comprised of a flexible, but substantiallyinextensible, dielectric film, preferably formed from a polymericmaterial, such as KAPTON™ “E” available from DuPont Chemical Corp., ofan approximate thickness between 25 microns and 75 microns. Further, thedielectric film 100 typically has conductive terminals 140 on its secondsurface.

[0027] A porous layer, such as a layer of a plurality of compliant pads110, is attached to the first surface of the film 100. The compliantpads 110 are typically made of an curable liquid elastomer material,such as the Dow Corning silicone elastomer 577, known as “Sylgard™”,with about 5-10% of fumed silica in order to obtain a stiff, thixotropicconsistency. The height of the compliant pads 110 is nominally uniform,as defined by the measurement from the base of the pads, at the firstsurface of the film, to the apexes of the pads. However, exactuniformity in height is not critical, as discussed below. The ratio ofwidth to height of each pad 110 is desirably about two to one (2:1) orgreater in order to maintain good pad structural integrity. Each paddesirably has width or diameter, measured at its juncture with firstsurface of the film 100, between about 300 μM, to 2000 μM. The pads arearranged in a grid pattern with center-to-center distance or “pitch”greater than the minimum width of each pad and preferably greater thanthe maximum width of each pad. Thus, each pair of mutually adjacent padsdefines a channel 117 between them, and all of the channels areinterconnected with one another to define a substantially continuousgrid of channels 117. The pad pitch and hence the width of the channels117 must be large enough to allow a compliant filler material 170 to beinjected into the channels 117, as described in greater detail below.

[0028] As shown in FIGS. 3A and 3B, the compliant pads 110 are typicallyformed and attached to the dielectric film 100 using a stencil mask 80having top and bottom surfaces and further having a plurality of holes90 extending therethrough. In a preferred embodiment, the bottom surfaceof the mask 80 is placed atop the first surface of the film 100 andcurable liquid elastomer is wiped or screened across the first surfaceof the mask 80 so that the elastomer is deposited into each of the holes90. If the elastomer is viscous or thixotropic enough, the mask is thentypically removed and the pads are cured or partially cured by heatingor exposing them to ultra violet light or a combination thereof, such asby exposing the elastomer to ultraviolet light to create a “skin” ofcured elastomer and then curing or partially curing the elastomer by aheating step. In the case of Dow 577 elastomer, the elastomer istypically heated to approximately 125° C. for 40 minutes.

[0029] The mask 80 may also be removed after the curing step. In thiscase, the elastomer will conform to the shape of the hole 90 in the maskas it begins to cure. Thus, the pads 110 in FIG. 3A will be cylindricalin shape with substantially flat top surfaces. If the mask 80 is leftatop the dielectric film 100 during the curing step, the pads maypartially adhere to the mask 80 at an exposed top edge of each hole 90on the first surface of the mask as the elastomer cures. This makesremoval of the mask 80 while maintaining the integrity of the pads 110difficult. This problem can be effectively dealt with by using analternate stencil mask 85, shown in FIG. 3B, where each hole in the maskhas a smaller top edge circumference on the top surface of the mask 85than on the bottom surface of the mask 85. The smaller circumference ofeach hole reduces the strength of the attachment of the compliant pads110 to the mask 85. However, because of the shape of enclosure 95 of theembodiment shown in FIG. 3B, the liquid elastomer typically must bescreened across the first surface of the mask in two directions toensure the enclosure 95 is completely filled. The elastomer will thentake on a rounded shape corresponding the shape of the enclosure 95 andwill retain its shape after curing. The Dow 577 elastomer, discussedabove, also shrinks by approximately 5% during the curing step therebyfacilitating the removal of the mask 85. Another solution to thisproblem would be to coat the mask 80 with a material that will resistthe adherence of the elastomer as it cures.

[0030] Returning to FIG. 1, a semiconductor chip 120 is next abutted tothe compliant pads 110. The chip 120 has a face surface bearing contacts130 and a back surface. The face surface of the chip is aligned with thecompliant pads so that the none of the pads 110 cover the contacts onthe chip. In the next step of the operation, the dielectric film 100 andthe second support structure or chip 120 are squeezed together between afirst platen 107 and a second platen 127. The platens are urged towardone another by a press (not shown). The first platen bears on theterminals 140 of the film 100, and forces these terminals intosubstantially coplanar alignment, whereas the second platen bears on theback surface of chip 120. During this process, the pads 110 arecompressed. The extent of such compression varies from pad to pad,depending upon the original height of each pad, the heights of thevarious terminals 140 and other tolerances. Because the pads canaccommodate variations in this manner, the heights of the pads prior tocompression need not be precisely uniform. Some particularly short pads,such as pad 115, may not contact the chip surface even aftercompression. This situation is acceptable provided that the remainingpads substantially support the terminals 140 against the first platen.

[0031] Where the pads 110 are only partially cured during the padforming process, described above, they are tacky and adhere to the chip120 when it is compressed thereagainst. In a preferred embodiment, thechip 120 would be heated to about 100° to 180° C. and then compressedagainst the pads 110 so that the partially cured pads 110 fully cure dueto the heat of the chip 120 and in so doing adhere to the face surfaceof the chip. Alternatively, the apex of each pad may be made tacky byproviding an adhesive, a silicone gel, or a film of uncured siliconeelastomer thereon so that the chip 120 adheres thereto. Similarly, thechip surface may be coated with an adhesive in the regions remote fromthe contacts 130 so that the tips of the pads 110 engage such adhesive.While adherence of the pads 110 to the chip 120 helps maintain thealignment between the chip 120 and the film 100, it is not imperativethat the pads 110 are adhered to the chip 120 so long as the chip can beheld in place by compressing it against the pads 110. The heat andpressure applied during this step of the process permanently deform or“set” the pads to the heights and shapes achieved when the film and chipare squeezed between the platens. Once set in this manner, the pads tendto retain the film and chip in the same relative positions as achievedduring the squeezing step, and thereby tend to retain the terminals 140in coplanar alignment with one another.

[0032] The contacts 130 on the chip 120 are then attached to theterminals 140 on the film 100 through any suitable means. In thisembodiment, each contact 130 is electrically connected to one terminal140 through a flexible lead 150 extending therebetween. The terminal 140is comprised of a conductive blind via (a via extending from the secondto the first surface of the film 100 and having a closed bottom surfacesubstantially in a plane with the first surface of the dielectric film100) and a solder ball (shown in dashed lines), which is typicallyattached after the package is complete.

[0033] As shown in FIG. 2, a low viscosity compliant filler 170 is thendisposed between the chip 120 and the dielectric film 100 through thechannels between the adjacent pads 110. An injection nozzle 160 isdisposed adjacent an edge of the film 100. The filler 170 passes throughthe nozzle into the channels 117. The compliant filler 170 may becomprised of an elastomer, a gel or a compliant thixotropic material solong as the elastomer, gel or material retains its compliancy. Oneexample of such a filler is Dow Corning elastomer Q1-4939. In apreferred embodiment, the filler 170 is comprised of a curable liquidelastomer having a viscosity adapted to facilitate flow of the liquidinto the channels 117 between the adjacent pads 110. An evacuatedenvironment can also be provided during the elastomer flow so that voidsin the liquid elastomer caused by trapped air may be eliminated.Capillary action tends to pull the low viscosity liquid elastomer 170into the channels 117 as the elastomer wets the pads, the chip and thefilm. The liquid elastomer 170 tends to remain between the chip and thesubstrate because of the surface tension forces. If necessary, however,the edges of the assembly may be closed by any suitable means to trapthe liquid 170. During this process, the liquid elastomer alsoencapsulates leads 150. The liquid 170 is then cured, typically byheating the assembly while it is held between flat opposing surfaces,such as the platens 107, 127 used for the compression step. This resultsin a planar, compliant interface between the chip 120 and the dielectricfilm 100. The resulting assembly may then be freed to form an individualsemiconductor chip package.

[0034] It is important to understand that the function of the compliantpads is to provide a uniformly supported, planar standoff between thechip and the dielectric film, while allowing the compliant filler to beinjected between the adjacent pads 110 to create a compliantencapsulation layer having a controlled thickness. So long as there isuniformity of support, it does not matter whether the pads 110 are indirect registration with single or multiple terminals 140, as shown inFIGS. 4A and 4B, or whether the pads 110 are in random registration winrespect to the terminals 140, as shown in FIG. 4C. It also does notmatter whether all of the pads in a common array are of the same generalshape or dimension.

[0035]FIG. 5A shows a top plan of a preferred embodiment of the firstsupport structure and the compliant pads. Tape 200 is a continuouselongated strip of film with handling and locating features such assprocket holes 210 adapted for engagement by tape-handling equipment ofthe type commonly employed in tape automated bonding or “TAB” processingin the semiconductor industry. Tape 200 includes a plurality of regions201 spaced apart along its length, each such region constituting asupport structure for one chip. Each region has pads 220 disposed in agrid on its first surface so that the pads define channels 203therebetween, and has terminals 207 on its second surface, the terminalsbeing connected to leads 225 extending across an aperture or “bondwindow” 215 in the tape, the bond window being in the form of a slotextending around a rectangular loop. The leads are adapted for bondingto the contacts of the chip. For example, each lead may have adetachable section as disclosed in International Patent Publication WO94/03036, the disclosure of which is hereby incorporated by referenceherein. The terminals and leads can be arranged in a “fan out” pattern,wherein the terminals are disposed on that portion of the tape outsideof the loop defined by the bond window; in a “fan in” pattern whereinthe terminals are disposed on that portion of the tape encircled by thebond window; or both. Such a tape can be mass-produced, to includenumerous regions.

[0036] In use, a series of chips can be bonded to the various regions ofthe tape by abutting each chip against the compliant pads of one regionand compressing and bonding the chip and pads in the same manner asdiscussed above. Each pad 220 may be only partially cured, or else mayhave a tacky layer 230 (FIG. 5B) of adhesive, silicone gel or uncuredsilicone elastomer on its apex, so that the pads will adhere to thechip. Alternatively or additionally, the chip may have such a tackylayer on its first surface in areas remote from the contacts. During theabutting step, the tape is juxtaposed with each chip so that one regionof the tape overlies the chip, and the contacts of each chip are broughtinto coarse alignment with the leads of the associated region of thetape, using conventional machine-vision alignment techniques. Thisalignment is maintained during the compressing and pad bonding steps.After the pads have been bonded to the surfaces of the chip, the bondedpads maintain the alignment of the leads and contacts. The terminals ofeach region are electrically connected to the contacts of the associatedchip, as by advancing a bonding tool into the bond window to engage eachlead and connect it to the aligned contact of the chip.

[0037] Preferably, the bonding tool captures the bond region of the leadand brings it into more precise alignment with the chip contact. Afterbonding the leads, the compliant filler is introduced into the channels203 between the pads, in substantially the same way as described above.Typically, the film 200 and the chip are compressed while the liquidmaterial is introduced. These operations may be performed at separatestations of a processing line. At any time after the pads of a regionhave been bonded to a chip, the chip can be transported to the nextstation of the processing line by moving the tape using the sprocketholes 210. Each region of the tape can be severed from the remainder ofthe tape after all of the process steps have been completed to providean individual chip and film assembly. Alternatively, each region can besevered before the final process step, and a portion of the process canbe completed after severance. In a further embodiment of the invention,a plurality of individual regions of the tape may remain connected toone another.

[0038] Typically, each of these leads 225 will be detached within ornear an edge of the bonding window 215 when such lead is connected to arespective chip contact pad. If the leads and the terminals are arrangedin a fan-out pattern, this action detaches the tape center portion 205,generally centered within the loop defined as the bonding window 215from the outer portions of the tape. However, the compliant pads 220disposed on the tape center portion 205 provides support for the leads225 during the bonding phase. Conversely, in a fan-in arrangement, theexterior portion of each region may optionally be removed afterconnecting the leads.

[0039] In the alternative embodiment shown in FIG. 5C, a plurality ofindividual regions 201 of the tape remain together and are not severedfrom one another during the process. A short length of tape thus retainsa plurality of chips side-by-side, as a multi-chip module. Thus, asillustrated in FIG. 5C, the back surfaces of these chips or modules maybe mated to heat spreader/sinks 270 before or after the step of cuttingthe short length of tape bearing the chips free from the remainder ofthe tape. Further, a thermally conductive die attach material may beused between the chips 240 and the heat spreader 270 to aid in thetransfer of heat from the chips. Such die attach materials are widelyused in the semiconductor industry.

[0040] In a further alternative arrangement, a plurality of individualregions, each adapted to form a support structure for an individualchip, are formed side-by-side in a two-dimensional array on a unitaryfilm in the shape of a panel. Plural chips are attached to these regionsin the same manner as discussed above, so as to form a multi-chipassembly. In either arrangement where plural chips are employed, thesteps of abutting and compressing the pads and injecting the liquidcompliant filler can be performed simultaneously for all of the chips,using platens large enough to accommodate the entire array, or seriatimso that each chip is processed separately.

[0041]FIG. 6 shows a side view of a compliant interface, as described inreference to FIGS. 1 and 2, being formed between a second supportstructure or semiconductor wafer 350 having contacts (not shown) on afirst surface and a first support structure or film 300. The wafer 350includes an integral plurality of individual chips, each defining aportion of the first surface, and each including a plurality of contactsthereon. The film 300 has terminals 305 on its second surface, withleads (not shown) connected to the terminals 305. The terminals 305 andto leads are disposed in a plurality of regions, each corresponding toone chip. Each region of the film may include a bond window (not shown),as discussed above.

[0042] Before engagement with the wafer 350, compliant pads 320 definingchannels 325 therebetween are formed on the first surface of the film.Here again, each pad 320 may be only partially cured, or may have acurable tacky material on its apex. The film 300 is stretched taut andbonded to a rigid ring structure 310. The film 300 is then aligned withthe wafer so that each region of the film is aligned with one chip ofthe wafer, and so that the leads are aligned with the contacts. Thewafer is then abutted against the compliant pads 320. A platen 360 isapplied on the back surface of the wafer 350. Another platen 370 isapplied on the second surface of the film so as to engage the terminals305. In the same manner as described above, the platens are forcedtoward one another, thereby compressing pads 320 and bringing thecontacts 303 into substantially coplanar alignment with one another.While the contacts and pads are held in this position, the pads may befully cured or set by heating the platen 360/wafer 350, thereby securingthe film in position on the wafer. Alternately, a tacky adhesive may beapplied to the first surface of the wafer around the contacts thereon toensure the adherence of the pads 320 to the wafer 350. After thisoperation, the leads are bonded to the wafer contacts, followed byinjection of the liquid curable elastomer 340 into the channels 325.While the contacts are again held in position by the platens, the liquidelastomer is cured to form a complete assembly encompassing the entirewafer and all regions of the film.

[0043] Individual, fully encapsulated parts may then be cut from thewafer. These separated parts will have a compliant interface toaccommodate for the thermal coefficient of expansion mismatch betweenthe parts and their eventual supporting substrates, such as printedwiring boards. Further, the encapsulant protects the individual partsfrom moisture and contaminants.

[0044]FIG. 7 shows a further embodiment having an array of compliantpads 410 held in place using a holding element 400. The pads 410 arethen fixably placed atop either the contact bearing surface of asemiconductor chip (second support structure) or a flexible dielectricfilm (first support structure) typically by placing a tacky adhesiveonto one of those surfaces. The holding element 400 is then removed.Desirably, the holding element 400 is made of a material which willbreak apart or peel off the pads easily so that the adherence of thepads 410 to either the wafer or the film is not comprised. The pads 110,as shown in FIG. 1, are not typically screened across and cured to thecontact bearing surface of the chip for fear that the contacts will becontaminated by stray uncured elastomer. In the embodiment shown in FIG.7, however, the pads are being attached after they have been eitherfully cured or partially cured to the point that they may retain theirindividual structural integrity. Thus, the pads 410 may be accuratelyplaced and typically adhered on the chip's surface without anysubstantial fear that the chip's contacts will be contaminated. The padsare then compressed between the chip and the supporting substrate usingplatens and liquid elastomer is next injected into the channels betweenthe adjacent pads and cured, as described more fully in relation to theabove embodiments.

[0045] The embodiment shown in FIG. 8 shows a side view of an alternateembodiment in which the pads 510 are compressed between a second supportstructure which includes a heat spreader 520 having an indentation onits first side and a first support structure 500, which includes aflexible dielectric film. A chip 530 is disposed within the indentationof the heat spreader 520 so that the front or contact-bearing surface580 of the chip faces outwardly at the opening of the indentation, andthus faces outwardly from the first surface 570 of the heat spreader.Dielectric film 500 has a bond window 550, terminals 590 on one surfaceand leads 595 connected to the terminals 590. After pads 510 areprovided on the dielectric film 570 or the heat spreader by any of thetechniques discussed above, the dielectric film 500 and the heatspreader 520 are aligned so that the outer portions of the dielectricfilm overly the first surface 570, and so that the bond window 550 isaligned with the contacts on the chip. The leads 595 are bonded to thecontacts of the chip. After bonding the leads, the bond window is closedby a sheet 540 of a further dielectric material such as a solder maskmaterial which adheres to film 500 and thus forms a continuation of thefilm 500. Typically before bonding the leads, the pads 510 arecompressed against first surface 570 by engaging the heat spreader 520and the film 500 between a pair of platens (not shown) so that one suchplaten bears on the rear surface of the heat spreader, opposite fromsurface 570 and so that the other platen bears on terminals 590. Thiscompression brings the terminals into coplanar alignment. As describedabove, the liquid compliant filler 560 is injected into the channelsbetween the pads and cured while the platens hold the system incompression. In this embodiment, the pads are not provided at theinterface between sheet 540 and the chip 530. If a solder mask 540, orother suitable containing means, is placed over the bonding window 550,a compliant filler material 560 may be injected between the adjacentpads 510 creating a planar, compliant interface while simultaneouslyencapsulating the chip 530.

[0046] Having fully described several embodiments of the presentinvention, it will be apparent to those of ordinary skill in the artthat numerous alternatives and equivalents exist which do not departfrom the invention set forth above. It is therefore to be understoodthat the present invention is not to be limited by the foregoingdescription, but only by the appended claims.

[0047] Although the present invention has been described with referenceto particular preferred embodiments, it is to be understood that theembodiments are merely illustrative of the principles and application ofthe present invention. It is therefore understood that numerousmodifications may be made to the preferred embodiments of the presentinvention without departing from the spirit and scope of the presentinvention as defined by the claims.

1. An assembly comprising: (a) a structure; (b) a plurality ofterminals; and (c) a plurality of compliant pads disposed between saidterminals and said structure, said terminals being aligned with at leastsome of said pads, said pads providing a standoff between said structureand said terminals.
 2. An assembly as claimed in claim 1 wherein saidstructure includes at least one semiconductor chip, said terminals beingconnected to said at least one chip.
 3. An assembly as claimed in claim2 wherein said at least one semiconductor chip includes a firstsemiconductor chip having a face surface bearing contacts, saidcompliant pads being disposed between said terminals and said facesurface of said first semiconductor chip, said terminals being connectedto said contacts of said first semiconductor chip.
 4. An assembly asclaimed in claim 2 wherein said structure includes a plurality ofsemiconductor chips.
 5. An assembly as claimed in claim 2 wherein saidstructure includes a unitary wafer incorporating a plurality ofsemiconductor chips.
 6. An assembly as claimed in claim 2 furthercomprising flexible leads extending between said terminals and saidchip, said terminals being connected to said chip through said flexibleleads.
 7. An assembly as claimed in claim 1 further comprising acompliant filler disposed between said compliant pads.
 8. An assembly asclaimed in claim 1 further comprising a flexible film, said terminalsbeing disposed on said film, said compliant pads being disposed betweensaid film and said structure.
 9. An assembly as claimed in claim 1wherein said pads are arranged in a grid pattern.
 10. An assembly asclaimed in claim 9 wherein said pads are spaced apart from one anotherso as to define channels between adjacent ones of said pads.
 11. Anassembly as claimed in claim 10 wherein the center-to-center distancebetween adjacent ones of said pads is greater than the minimum width ofeach said pad.
 12. An assembly as claimed in claim 10 wherein thecenter-to-center distance between adjacent ones of said pads is greaterthan the maximum width of each said pad.
 13. An assembly as claimed inclaim 1 wherein each said pad has a maximum width between about 300 μmand 2000 μm.
 14. An assembly as claimed in claim 1 wherein each said padhas a ratio of width to height of about two to one (2:1) or greater. 15.An assembly as claimed in claim 1 wherein said pads are formed from anelastomer.
 16. An assembly as claimed in claim 15 wherein said elastomeris a silicone elastomer.
 17. An assembly as claimed in claim 1 whereinat least some of said pads are multi-terminal pads, a plurality of saidterminals being aligned with each said multi-terminal pad.
 18. Anassembly as claimed in claim 1 wherein at least some of said pads aresingle-terminal pads, each said single-terminal pad being aligned withonly one of said terminals.
 19. An assembly as claimed in claim 1wherein said pads are of substantially uniform shape and size.
 20. Anassembly as claimed in claim 1 wherein said pads are of different shapesand sizes.